1. Field of the Invention
The present invention is directed generally to a process for separating hydrocarbon gas constituents and, more specifically, to a cryogenic process for separating components of natural gas.
2. Description of the Prior Art
Various cryogenic processes are known in the prior art for recovering ethane and heavier hydrocarbon components from multi-component gas streams including natural gas, refinery gas and synthetic gas streams, comprised primarily of methane. Natural gas usually has a major proportion of methane and ethane, with these two components comprising at least about 50 mole percent of the total gas volume. The gas may also contain relatively lesser quantities of heavier components such as propane, butanes, pentanes, and the like, as well as hydrogen, nitrogen, helium, carbon dioxide, ethylene and other gases.
The process of the present invention is primarily concerned with the recovery of ethylene, ethane, propylene, propane and heavier hydrocarbons from feed gas streams containing primarily methane of the type described. A typical gas stream might contain, for example, about 90 weight percent methane; about 5 weight percent ethane, ethylene and other C.sub.2 components; and about 5 weight percent heavier hydrocarbons such as propane, propylene, butanes, pentanes, etc. and nonhydrocarbon components such as nitrogen, carbon dioxide and sulfides.
Cryogenic processes have become popular in recent years for separating hydrocarbon gas constituents of the type described because of the availability of economical equipment that produces power while simultaneously expanding and extracting heat from the gas being processed. Such processes are now generally favored for ethane recovery, since they provide maximum simplicity with ease of start-up, operating flexibility, improved efficiency, safety and reliability.
In a typical prior art cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams in the process and/or with external refrigeration means such as a propane compression-refrigeration system. As the feed gas cools, liquids may be condensed and collected in one or more separators as high pressure liquids containing certain of the desired C.sub.2 + components. Depending on the richness of the gas feed and the amount of liquid formed, the high pressure liquids may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquid results in further cooling of the stream. The expanded stream, comprising a mixture of liquid and vapor is fractionated in a demethanizer column. In the demethanizer column, the expanded and cooled streams are stripped or distilled to separate residual methane, nitrogen and other volatile gases as overhead vapor from the desired C.sub.2 components, C.sub.3 components, and heavier components as a bottom liquid product.
In this discussion, the term "demethanizer" will be taken to mean any device that can remove methane from a feed gas, including what is often referred to as a "deethanizer", which is designed to remove both methane and ethane. Such devices will be understood by those skilled in the art to include devices capable of removing methane from feed gases by the application of heat, including distillation, rectification and fractionation columns or towers. The exact member of trays or levels used in such columns will be subject to overall design considerations, efficiencies and optimization considerations.
A number of techniques are used in the prior art processes to both satisfy the heat requirements of the demethanizer and extract refrigeration from the overall process. A typical practice in the prior art cryogenic expansion recovery processes is to split the incoming feed gas stream into two streams, both having the same composition as the feed stream either before or after initial cooling. One of the split streams is typically processed so as to take advantage of the heat transfer capabilities inherently possessed by the feed gas, which typically has a higher temperature than other streams in the process.
The vapor from one of the streams is typically passed through a work expansion machine (turboexpander), or through an expansion valve, to lower the pressure so that additional liquids are condensed as a result of the further cooling of the stream. The pressure of the stream after expansion is essentially the same as the pressure at which the distillation column is operated. In such cases, the combined vapor-liquid phase is usually supplied as feed to the column.
In other cases, a vapor portion of the incoming feed is cooled to substantial condensation by heat exchange with other process streams. The resulting cooled stream is then expanded through a conventional expansion device, such as an expansion valve, to the pressure of the demethanizer. During expansion, a portion of the liquid will vaporize, resulting in cooling of the stream. The flash expanded stream is then supplied as a top feed to the demethanizer column. Typically, the vapor portion of the expanded stream and the demethanizer column overhead vapors combine as a residual methane product gas.
Under ideal conditions, the residue gas leaving the demethanizer column would contain substantially all of the methane in the feed gas with essentially none of the heavier hydrocarbon components and the bottom fraction leaving the demethanizer column would contain substantially all of the heavier components with virtually none of the methane or more volatile components. Under actual operating conditions, this ideal situation is not realized and the methane product of the process includes other vapors leaving the top fractionation stage of the column. As a result, there can be a considerable loss of C.sub.2 components due to the fact that the top liquid feed contain substantial quantities of C.sub.2 components and heavier components, resulting in these components leaving the top fractionation stage of the demethanizer as vapor.
It is possible to reduce the loss of desirable components from the column by contacting the rising vapors within the column with a reflux (liquid) which, preferably, contains very little C.sub.2 components and heavier components. The return of a liquid reflux is desirable because it is the condensed liquid that increases the recovery percentage of the desired column bottoms product. Those skilled in the art will also appreciate that the reflux effect is optimized when the vapor recycle stream is totally or substantially condensed before expansion to the demethanizer operating pressure. Where a large portion of the reflux stream is still in the vapor state, the uncondensed vapor mixes with the residue gas in the demethanizer and both are discharged as overhead vapors, thereby decreasing product recovery. Preferably, the reflux is substantially condensed and is constituted so as to be capable of absorbing the majority of the C.sub.2 components and heavier components from the overhead vapors of the column.
Various attempts have been made to improve the above described prior art processes. These attempts are primarily directed toward increasing ethane recovery while reducing external energy usage. The present invention provides an improved cryogenic expansion recovery process for separating hydrocarbon gas constituents having certain advantages, as will be discussed in detail below. The process of the invention can also be used advantageously in combination with the prior art processes.
It is therefore an object of the present invention to provide a cryogenic separation process for separating hydrocarbon gas constituents which increases the recovery of the desired components.
Another object of the invention is to provide an enhanced reflux process while lowering external energy requirements.
Another object of the invention is to provide such a process in which a reflux stream is returned to the top of the demethanizer column for increased ethane/propane recovery in the column bottoms product.
Another object of the invention is to provide a recycle stream that is substantially totally condensed, thereby maximizing the recovery of ethane/propane.
Another object is to reduce the recycle stream equipment required to provide the same amount of liquid reflux to the top of the demethanizer as is currently accomplished by existing schemes.
Another object of the invention is to reduce the number of expander-compressors and other equipment needed.
Another object of the invention is to utilize the lean residue gas, to reboil the demethanizer gas column, having thus the advantage of minimizing heat exchanger "pinching" due to gas richness.
Another object of the invention is to provide means to existing units using the existing conventional "split feed" process to retrofit their units to a high recovery process.
Another object of the invention is to utilize hot residue gas to reboil the demethanizer. Even when hot residue gas was used in other prior art schemes, it was never utilized as a part of recycle/reflux scheme as in Applicant's claimed invention.